Intravascular optical coherence tomography system with pressure monitoring interface and accessories

ABSTRACT

An optical coherence tomography system and method with integrated pressure measurement. In one embodiment the system includes an interferometer including: a wavelength swept laser; a source arm in communication with the wavelength swept laser; a reference arm in communication with a reference reflector; a first photodetector having a signal output; a detector arm in communication with the first photodetector, a probe interface; a sample arm in communication with a first optical connector of the probe interface; an acquisition and display system comprising: an A/D converter having a signal input in communication with the first photodetector signal output and a signal output; a processor system in communication with the A/D converter signal output; and a display in communication with the processor system; and a probe comprising a pressure sensor and configured for connection to the first optical connector of the probe interface, wherein the pressure transducer comprises an optical pressure transducer.

FIELD OF INVENTION

This invention relates generally to invasive medical devices and morespecifically to fiber-optic systems for imaging the lumen of bloodvessels and for measuring physiological variables such as bloodpressure.

BACKGROUND

The functional severity of a stenotic lesion in an artery can beassessed by measuring the pressure gradient across the lesion.Intravascular pressure measurement, particularly in the coronaryarteries, has gained widespread acceptance as a tool for guidingcatheter-based interventional procedures. Angioplasty or stenting oflesions in coronary arteries can be avoided if the fractional flowreserve (FFR), defined as the ratio of the blood pressures measureddistal to and proximal to a lesion after injection of a vasodilatingdrug, exceeds a certain clinically defined threshold.

Various devices have been developed for sensing arterial pressure at thetip of miniature catheters during medical diagnostic and interventionalprocedures. The most widely used device of this type, often referred toas a “pressure wire,” employs an electronic pressure transducer embeddedin the side of a long metallic tube through which electricallyconducting wires pass to a connector at the proximal end. Typically, thetransducer is mounted at a distance 1-2 cm proximal to a spring at thedistal tip of the tube. The operator navigates the tube through theartery manually until the transducer reaches the desired location forlocal pressure measurement.

One drawback of electronic pressure measurement systems is therelatively large minimum diameter of the pressure wire, which isdetermined by the size of the transducer, wires, and wire attachmentassembly. The diameter of a pressure wire is critically important,because it must pass through narrow stenoses in blood vessels withoutsignificantly increasing the pressure gradient across the stenosis orpreventing passage of the wire through the stenosis. This is especiallysignificant because diseased arteries that are candidates forangioplasty, for example, can have lumen diameters smaller than 1 mm.

A second drawback of electronic pressure monitoring systems is theirsusceptibility to electrical interference and calibration drift. Carefulsealing of the wires and transducer to avoid moisture intrusion andshielding of the wires against electromagnetic interference are requiredto minimize environmental disturbances.

Frequently, acquisition of intravascular images and measurement ofintravascular pressures during a single medical procedure is desirable.However in such an application, when intended to be used with imagingcatheters, electronic pressure wires, because of their wire connections,are difficult to integrate with intravascular imaging catheters.

The present invention addresses these issues.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for providingcost-effective pressure monitoring capabilities to an intravascularoptical coherence tomography (OCT) system. The combined system permitsconvenient use of both modalities from a single system console in whichprocessing, catheter control, and parameter and image display arecontrolled by software executing on the same computer.

The invention provides, in part, an OCT system with integrated pressuremeasurement. The OCT system in one embodiment includes: aninterferometer, an acquisition and display system, and a probe includinga pressure sensor. The interferometer in one embodiment includes: awavelength swept laser; a source arm in optical communication with thewavelength swept laser; a reference arm in optical communication with areference reflector; a first photodetector having a signal output; adetector arm in optical communication with the first photodetector; aprobe interface; and a sample arm in optical communication with a firstoptical connector of the probe interface. The acquisition and displaysystem in one embodiment includes: an analog to digital converter havinga signal input in electrical communication with the first photodetectorsignal output and a signal output; a processor system in electricalcommunication with the analog to digital converter signal output; and adisplay in electrical communication with the processor system. The probein one embodiment is configured for optical connection to the firstoptical connector of the probe interface, and the pressure transducerincludes an optical pressure transducer.

In some embodiments of the OCT system, the analog to digital converterfurther includes a sample clock input and a trigger input, and the OCTsystem further includes: a power splitter having a first arm in opticalcommunication with the wavelength swept laser, a second arm in opticalcommunication with the source arm of the interferometer, and having athird and forth arm; a trigger generator in optical communication withthe third arm of the power splitter, and having a trigger output; and asample clock generator in optical communication with the forth arm ofthe power splitter and having a sample clock output. The trigger outputof the trigger generator and the sample clock output of the sample clockgenerator is in electrical communication with the trigger input andsample clock input of the analog to digital computer, and the analog todigital converter can convert a signal from the first photo detector inresponse to a trigger signal from the trigger generator and a sampleclock signal from the sample clock generator.

In some embodiments, the OCT system further includes an optical switchin optical communication between the reference arm and the referencereflector.

In some embodiments, the probe of the OCT system further includes an OCTimaging optical system.

In some embodiments, the OCT system includes: a second light source; aspectrometer having an optical input and an electrical signal output; anoptical circulator having a first arm in communication with the secondlight source, a second arm in optical communication with thespectrometer optical input, and a third arm; and a wavelength divisionmultiplexer in optical communication between the sample arm of theinterferometer and the probe interface and having a third arm in opticalcommunication with the third arm of the optical circulator, where theelectrical signal output of the spectrometer is in electricalcommunication with the processor system.

In some embodiments of the OCT system, the analog to digital converterhas a second signal input; the power splitter further includes a fourtharm; the probe interface further includes a second optical connector;and the OCT system further includes: a second photodetector, the secondphotodetector including an electrical signal output and a optical signalinput and a circulator. The circulator includes: a first arm in opticalcommunication with the fourth arm of the power splitter; a second arm inoptical communication with the optical input of the secondphotodetector; and a third arm in optical communication with the secondoptical connector of the probe interface, and the electrical signaloutput of the second photodetector can be in electrical communicationwith the second signal input of the analog to digital converter.

In some embodiments of the OCT system, the circulator is a multimodecirculator and the third arm of the circulator is a multimode fiber; andthe optical coherent tomography system further includes a single mode tomultimode converter optically connected between the power splitter andthe multimode circulator. The fourth arm of power splitter includes asingle mode optical fiber, and the first arm of the circulator includesa multimode optical fiber.

The invention also provides, in part, a probe for an OCT system. Theprobe in one embodiment includes: a body defining a bore and having afirst end and a second end; an optical fiber located within the bore,the optical fiber having a first end and a second end; an opticalpressure transducer located within the bore and in optical communicationwith the first end of the optical fiber; and a fiber optic connector,located at the second end of the body and in optical communication withthe second end of the optical fiber, where the body further defines atleast one opening from the bore to the environment by which pressurefrom the environment is transmitted to the optical pressure transducer.In some embodiments, the second end of the optical fiber includes afiber optic ferrule.

In some embodiments, the probe further includes a spring tip positionedat the first end of the body.

In some embodiments, the probe further includes a removable torquehandle removably attached to the body.

In some embodiments of the probe, the fiber optic connector defines abore and includes a mating unit sized and configured to receive thefiber optic ferrule, and the fiber optic connector further includes alocking clamp to removably attach the body to the fiber optic connector.

In some embodiments of the probe, the body further includes a guidehaving a first end and a second end, the guide positioned at the firstend of the body and defining a second bore, the second bore sized andshaped to permit a guide wire to enter the guide through a first openingin the first end of the guide and to pass through the second bore andout through a second opening in the guide. In some embodiments, theoptical fiber and the optical pressure transducer are movable within thebore. In some embodiments, the optical fiber passes through a liquidseal located in the bore adjacent the fiber optic connector.

The invention also provides, in part, a combination probe for an OCTsystem. The combination probe includes: a body having a wall defining abore and having a first end and a second end; an optical fiber locatedwithin the bore, the optical fiber having a first end and a second end;a partial reflector located within the bore and positioned to reflect afirst portion of light received from the first end of the optical fiberfrom through the wall of the body; an optical pressure transducerlocated within the bore and positioned to receive a second portion oflight from the first end of the optical fiber; and a fiber opticconnector, located at the second end of the body and in opticalcommunication with the second end of the optical fiber, where the bodyfurther defines at least one opening from the bore to the environment bywhich pressure from the environment is transmitted to the opticalpressure transducer.

The invention also provides, in part, a method of determining pressurein a vessel as measured by an optical pressure transducer in an OCTsystem which includes an interferometer having a photodetector locatedin a detector arm of the interferometer and having an optical pressuretransducer located in the sample arm of the interferometer. The methodincludes the steps of: sampling a signal from the photodetector to forma sampled pressure signal; normalizing the sampled signal to obtain anormalized sampled pressure signal; removing cavity noise to form acleaned normalized sampled pressure signal; finding a minimum value inthe cleaned normalized sampled pressure signal; and tracking the minimumvalue of the cleaned normalized sampled pressure signal. In someembodiments of the method, the minimum value is determined by one ofconvolution, differentiation and gradient searching. In someembodiments, the method further includes the steps of: inserting acatheter having an optical pressure transducer into a vessel; and movingthe optical pressure transducer within the catheter.

The invention also provides, in part, a method of obtaining an OCT imagein a blood vessel using an OCT/pressure probe system. The methodincludes the steps of: inserting a combination OCT/pressure probecatheter into the blood vessel; setting the OCT/pressure probe system tomeasure pressure; determining the pressure drop across a putativestenotic region of the vessel; setting the OCT/pressure probe system toimage; and taking an OCT image of the putative stenotic region.

This Summary is provided merely to introduce certain concepts and not toidentify any key or essential features of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be understood morecompletely by referring to the drawings described below and theaccompanying descriptions.

FIG. 1 illustrates an embodiment of the design of an optical pressuretransducer known to the prior art that is suitable for use with theFourier-domain optical coherence tomography system.

FIG. 2 is a block diagram of an embodiment of an FD-OCT imaging systemthat is capable of collecting data from an optical pressure sensor orOCT imaging probe according to the present invention.

FIG. 3 is a block diagram of another embodiment of an FD-OCT imagingsystem that is capable of collecting data from an optical pressuresensor or OCT imaging probe according to the present invention.

FIG. 4 is a block diagram of yet another embodiment of an FD-OCT imagingsystem that is capable of collecting data from an optical pressuresensor according to the present invention using a multi-mode opticalpressure probe.

FIG. 5 is a block diagram of still yet another embodiment of an FD-OCTimaging system that is capable of collecting data from an OCT imagingcatheter and an optical pressure sensor operating in different opticalwavelength bands.

FIG. 6 a is a cross-sectional view of the distal end of an embodiment ofa steerable intravascular pressure probe constructed in accordance withthe invention.

FIG. 6 b is a cross-sectional view of the proximal end of a steerableintravascular pressure probe of FIG. 6 a.

FIG. 6 c is a cross-sectional view of the distal end of an embodiment ofthe probe in which the tube diameters proximal and distal the transducerare smaller than the tube diameter adjacent the transducer.

FIG. 7 a is a cross-sectional view of the distal end of an embodiment ofa steerable intravascular pressure wire with a detachable opticaladapter constructed in accordance with the invention.

FIG. 7 b is a cross-sectional view of the proximal end of a steerableintravascular pressure wire with a detachable optical adapter of FIG. 7a.

FIG. 8 a is a cross-sectional view of the distal end of an embodiment ofa rapid-exchange intravascular catheter with a single pressure sensingport constructed in accordance with the invention.

FIG. 8 b is a cross-sectional view of the proximal end of arapid-exchange intravascular catheter with a single pressure sensingport of FIG. 8 a.

FIG. 9 a is a cross-sectional view of the distal end of an embodiment ofa rapid-exchange intravascular catheter with multiple pressure sensingports.

FIG. 9 b is a cross-sectional view of the proximal end of arapid-exchange intravascular catheter with multiple pressure sensingports of FIG. 9 a.

FIG. 9 c is a section through A-A′ of the embodiment of FIG. 9 b.

FIGS. 9 d-f show a schematic diagram of an embodiment of a multi-holeprobe and graphs of various pressure measurements made by the probe atvarious positions in a stenotic vessel.

FIG. 10 is a cross-sectional view of the distal end of an embodiment ofa rotational OCT imaging intravascular catheter that also functions asan intravascular pressure monitor.

FIG. 11 shows examples of waveforms recorded from an optical pressuresensor by an optical coherence tomography system operating in thepressure-sensing mode.

FIG. 12 is an example of a calibration curve of an optical pressuresensor.

FIG. 13 compares dynamic pressure reading obtained from an electronicpressure sensor and an optical pressure sensor configured to operatewith an FD-OCT system in accordance with the present invention.

FIGS. 14 (a, b) show an example of dynamic pressure readings obtainedfrom an optical pressure sensor during pullback through a model of astenosed blood vessel and the corresponding region in the vessel atwhich the readings were taken.

DETAILED DESCRIPTION

The following description refers to the accompanying drawings thatillustrate certain embodiments of the invention. Other embodiments arepossible and modifications can be made to the embodiments withoutdeparting from the spirit and scope of the invention. Therefore, thefollowing detailed description is not meant to limit the presentinvention. Rather, the scope of the present invention is defined by theclaims.

The present invention arises from the realization that the basicarchitecture of a frequency-domain OCT (FD-OCT) system, configured in aspecific manner, permits a user to make an intravascular blood pressuremeasurement when used in conjunction with fiber-optic Fabry-Perotpressure transducers. FIG. 1 illustrates the basic structure of aFabry-Perot optical pressure transducer 10, known to the prior art, thatis compatible with the combined OCT imaging/pressure measurement systemdisclosed herein. A Fabry-Perot cavity 14 is formed by the surfaces of adiaphragm 18 and the sensor body 22 and has a reflectivity that dependson optical frequency or wavelength of the light exiting the opticalfiber 26.

The depth of the cavity lies typically within the range of 1-20 μm andits width, which is limited by the diameter of the body of the sensor,lies typically in the range of 0.15-0.4 mm. Light from an optical fiber26 impinges on the cavity 14 and the same fiber 26 collects thereflected light as the diaphragm 18 flexes in response to externalpressure variations. When the sensor 10 is excited by a laser, theoptical signals returning from the cavity 14 through the optical fiber26 combine and generate a common-mode interference signal. An FD-OCTsystem, configured according to the present invention, performs thefunctions required to record these interference signals. Algorithms arediscussed below for processing the interference signals and displayingblood pressure waveforms.

One embodiment of an FD-OCT system 30 that is suitable for combined OCTimaging and blood pressure measurement according to the presentinvention is shown in FIG. 2. Because it performs both functions withlittle additional hardware, it is a simple cost-effective embodiment ofthe combined system. A wavelength-swept laser 34 (also referred to as an‘optical-frequency tunable laser’ or ‘swept-source laser’) emits nearlymonochromatic (line width <0.2 nm) near-infrared light within a specificspectral band suitable for intravascular OCT imaging. The most commonband of emission wavelengths lies within the telecommunications O band(1260≦λ≦1360 nm). The wavelength of the laser 34 is swept or steppedrapidly over a broad band of wavelengths. The sweep is completedtypically within a period less than 20 μs and repeats 10,000 times ormore each second. An optical power splitter 38 separates the light intothree channels. The first channel, which includes a fiber Bragg gratingfilter 42 (or equivalent wavelength-selective filter) with a narrowbandpass (typically <1 nm) and a trigger generator 46, provides a timingpulse to the main analog-to-digital converter 50 that triggers the dataacquisition. This channel produces a timing pulse at a predeterminedoptical frequency or wavelength (typically 238 THz or 1260 nm) when theoutput of the laser sweeps through the passband of the fiber Bragggrating filter. The timing pulse triggers the start of the acquisitionof the analog signals from the photoreceiver on each edge of the sampleclock.

Light in the second channel is routed through a single-mode opticalfiber to a sample clock generator 54, which includes a referenceinterferometer and associated electro-optics that generate sample clockpulses at fixed optical frequency intervals. The sample clock generatorensures that the signals from photoreceiver 94 are collected by the dataacquisition and display unit 102 synchronously with the sweep of thelaser 34 at known wavelength steps. A fiber-optic Mach-Zehnder,Michelson interferometer, optical etalon, or equivalent type ofinterferometer with a known optical path difference can serve as thereference interferometer of the clock generator 54.

Light in the third channel is conducted through a single-mode opticalfiber to the main interferometer 58 which splits the light intoreference 60 and sample arms 62. The light in the reference arm passesthrough an optical switch 64 to a reference mirror 66 that sets thezero-point optical delay for the imaging system and determines the depthinto the tissue at which the OCT imaging will take place. The light inthe sample arm 62 passes to a motorized fiber-optic rotary coupler 70 inthe probe interface 74. The probe interface 74 includes a connector 78which permits various probes (combined OCT imaging/pressure measuringcatheter 82, OCT imagining 86 or optical pressure measuring probe 90) toconnect to the sample arm 62 of the interferometer 58.

A motorized translation stage in the probe interface 74 enables thefiber-optic core of the catheter (82, 28, 90) inserted into a vessel topull back with a constant speed. The optical output of the maininterferometer 58 is converted by the photodetector 94 to electricalsignals representative of the interference signals from the sample 60and reference arms 62 of the interferometer 58. These electrical signalsare converted to digital signals by the analog to digital converter(A/D) 50 and processed and displayed on the display unit 98 of the dataacquisition system 102. Because the trigger generator 46 and clockgenerator 54 are synchronized, the absolute optical frequency of theinterference signal acquired by the data acquisition system 102 duringevery laser sweep can be determined from the number of sample clockpulses acquired after each trigger pulse from the trigger generator 46.The absolute frequency reference is provided by the fiber-Bragg grating,which indicates the starting optical frequency; subsequent steps occurat equal optical frequency intervals set by the sample clock generator.

In one embodiment of the invention shown in FIG. 2 operates either inthe OCT imaging mode, pressure mode, or combined OCT/pressure modedepending on whether an OCT imaging 86 catheter, pressure probe 90, orcombined OCT/pressure catheter 82 is attached to the probe interfaceconnector 78. The mode of operation can be selected manually orautomatically, depending on the system configuration. Manual selectionrequires the user to choose the operating mode from a software menu.Automatic selection can be accomplished in one of several ways.

In one embodiment with the motorized fiber-optic rotary coupler 70 in astationary position, data acquisition unit 102 initiates after eitherthe OCT imaging catheter 86 or pressure probe 90 has been inserted. Asoftware algorithm identifies the type of probe according to theinterference signal pattern detected by the main interferometer 58 andphotodetector 94 and loads the appropriate control and display software.Once the pressure measurement has been completed, the precise locationand the severity of the stenosis can be determined and the OCT imagingprocedure can begin.

In a second embodiment of the system to automatically determine whattype of probe is connected to the system, following insertion of thecatheter 86 or pressure probe 90, the system attempts to rotate themotorized fiber-optic rotary coupler 70. A torque sensor in the motor ofthe motorized coupler 70 measures resistance to rotation. Torqueexceeding a specific threshold indicates that a pressure probe 90, witha non-rotating proximal connection, is attached. Once insertion of apressure probe 90 has been detected, the motor disengages and theappropriate control and display software loads.

In a third embodiment, an encoded electrical or optical tag (e.g., barcode, wire-encoded electrical connector, RFID tag, flash memory chip) onthe proximal end of the OCT imaging catheter 86 or pressure probe 90 (orboth 82) is read by the system to identify the appropriate mode ofoperation. The tag can be read automatically by the probe interface 74when the probe is inserted or, alternatively, a handheld device can beemployed to read the marker from the body or package of the probe. Thismethod of probe identification has the advantage that additional factorycalibration data encoded in the markers can be read at the same time.

In addition to features that enable automatic software configuration,the system of FIG. 2 also contains features that enable automatichardware configuration once the operational mode has been determined. Inthe standard OCT imaging mode, optical switch 64 is actuated, allowinglight to reflect from the reference mirror 66. However, in thepressure-measurement mode, because the system records common-modeinterference signals and no reference light is required, the opticalswitch 64 is turned off, isolating the reference reflector 66 from thereference arm 60 of the interferometer 58. In addition, sincedifferential (balanced) photodetection is employed only during OCTimaging, the electronic balancing circuits in the photoreceiver 94 areswitched to the single-ended (unbalanced) mode during pressuremeasurements.

FIG. 3 is a block diagram of another embodiment 30′ of the invention inwhich separate optical connections are provided on the probe interface74′ for the OCT imaging catheter 86 and optical pressure probe 90. Thisconfiguration enables acquisition of blood pressure measurements with aseparate pressure probe 90 before, during, or after OCT imaging.Measurement of pressure during OCT imaging can be accomplished byinserting both probes in the blood vessel at the same time. Anadditional optical channel 112 from the power splitter 38′ directs lightto the pressure probe 90 through an optical circulator 116. The opticalcirculator 116 re-directs the light that returns from the pressure probe90 into a second photodetector 120 that generates the electronic signalcorresponding to interference signals from which the pressuremeasurements are derived. These signals are digitized by a secondchannel of the analog to digital converter 50′ and recorded by the samedata acquisition system 102′ used to record the OCT interferencesignals. In contrast to the first embodiment of the inventionillustrated by FIG. 2, no optical switch 64 is required in the referencearm 60 to isolate the reference arm 60 from the reference reflector 66(FIG. 2) during pressure measurement.

A third embodiment of invention 30″, illustrated in FIG. 4, is similarto that shown in FIG. 3, except a multi-mode (MM) fiber 124 is usedinstead of a single-mode (SM) fiber 112 to conduct light over part ofthe path from the laser 34 to the optical pressure probe 90. The largercore of the multi-mode (MM) fiber 124 facilitates fabrication of thepressure probe and reduces the tolerances for aligning the fiber-opticconnection at the probe interface 78′ to the proximal end of thepressure probe. Typically, a 50-μm or 62.5-μm diameter graded-index MMtelecommunications fiber may be used in this application. To reduceinter-modal interference noise, a passive SM-to-MM converter 128, may beemployed to launch light from the laser 34 into the MM fiber 124.

FIG. 5 illustrates a fourth embodiment 30′″ of the invention thatenables simultaneous acquisition of OCT images and pressure measurementsfrom a single fiber-optic catheter that contains both an OCT opticallens assembly and an optical pressure transducer as described below. TheOCT and pressure interference signals are detected in twonon-overlapping wavelength bands, for example, 1250-1350 nm and1500-1600 nm or 1000-1200 nm and 1500-1600 nm, respectively. Asuperluminescent diode 130 emits broadband light that combines with theOCT laser light in a wavelength-division multiplexer 134 after passingthrough an optical circulator 138. The combined light from the maininterferometer 58 sample arm 62 and the superluminescent diode 130travels in the same SM optical fiber to the tip of the catheter wherelight in the appropriate band (as described below in conjunction withthe probe) impinges upon and reflects from either the tissue or thepressure transducer

The reflected light is passed back down the fiber and separated againinto two wavelength bands by the wavelength division multiplexer 134.Light reflected by the tissue in first band enters the main OCTinterferometer 58 through the sample arm 62 and light reflected by thepressure transducer in the second band enters a spectrometer 40 againafter passing through the optical circulator 138. The spectrometer 140records the spectrum of the light reflected from the pressure transducerand transmits the spectral data to the processor and display system 98′over a digital interface.

FIGS. 6-10 illustrate the designs of pressure-sensing catheters that arecompatible with an FD-OCT system configured according to the variousembodiments of the invention. Each of the catheters contains a miniatureoptical pressure sensor at the tip of a small-diameter (80 μm- or 125μm-diameter) optical fiber. The diameters of the sensor and fibers aresmall enough to enable the fabrication of pressure-sensing catheterswith dimensions of coronary and neurovascular guide wires(0.010-0.014″).

FIGS. 6( a-b) show cross-sectional views of the proximal and distal endsof one of the embodiments of the optical pressure probe. The pressureprobe 90 includes of a long hollow flexible tube 150 or an assembly oftapered tubes with an optical transducer 10′ mounted at the base of aspring tip 154 located at the distal end of the probe. The optical fiber155 from the pressure transducer 10′ connects to a fiber-optic connector157 mounted on the proximal end of the probe. Pressure-sensing slots orholes 160 in the mounting collar 164 at the base of the spring tip 154transmit the blood pressure from the vessel in which the probe isinserted to the diaphragm 18 of the pressure transducer 10′.Alternatively the optical pressure transducer 164 is exposed to theblood flow directly. Preferably, the hollow body of the tube 150 iscomposed of a metal, metal alloy, or metal-braided polymer that givesthe tube sufficient resistance to compression and rigidity to torque,while maintaining a high degree of flexibility.

To minimize the restriction of flow caused by placement of the probeacross a tight stenosis in a blood vessel, the body of the probe at itsdistal end is fabricated typically with an outer diameter of0.010-0.018″ (0.25-0.46 mm). To position the pressure probe, theoperator inserts the probe through a guide catheter into the artery andsteers the probe to the target location using a torque handle 168located near the proximal end. In accordance with the design of theFD-OCT system of FIG. 2, operating in the pressure measurement mode, thefiber-optic rotary coupler 70 in the probe interface 74 rotatespassively. The tip of the pressure probe 90 can be steered to the targetlocation in the blood vessel by disconnecting the fiber-optic connectorfrom the probe interface to allow free rotation of the body of theprobe. After the pressure wire has been positioned, the fiber-opticconnector can then be re-inserted into probe interface to obtainpressure measurements. Alternatively, since the fiber-optic rotarycoupler in the probe interface rotates passively, the tip of thepressure probe can be steered without disconnecting the fiber-opticconnection. Fabricating the pressure probe with a longer body (˜2meters) facilitates this alternative steering method. If the probeinterface does contain a rotatable element, the proximal end of thetransducer may be made flexible between the removable torque handle 168and the angle polished fiber optic connector 157. This allows thepressure wire to be steered with a simpler probe interface unit.

It should be noted that the diameter of the probe need not be constantacross the transducer. FIG. 6 c depicts a cross-sectional view of theprobe of FIG. 6 a but in which the diameter of the probe varies. Thewidths distal (D) and proximal (D′) to the transducer 164 are less thatthe width necessary to encompass the transducer 164. This configurationfunctions because the wider diameter at the transducer 164 and opening160 is not located within the stenotic region when pressure is measured.Hence, the diameter of the vessel in the region outside the stenosis issufficiently large to permit pressure to enter through hole 160 withoutconstricting flow and generating an anomalous pressure reading.

FIGS. 7( a-b) show cross-sectional views of an alternative constructionof an optical pressure probe. Similar to the probe shown in FIG. 6, thedistal end of the probe shown here is constructed from a long flexiblehollow tube 150 or an assembly of tapered tubes with the pressuresensing port located at the base of its spring tip 154. However, theprobe shown in FIG. 7 b includes a disposable optical adapter 172 thatgives the operator access to the proximal end of the adapter. Thisconfiguration enables the pressure probe to serve as a primary guidewire over which a balloon catheter or other interventional device can beinserted into the artery. For compatibility with standard devicesemployed in coronary interventions, the pressure probe is fabricatedwith an outer diameter of less than 0.014″ (0.36 mm) over its entirelength. An end-polished fiber-optic ferrule 176, with an outer diameterapproximately equal to that of the body of the probe, mates preciselywith a similar fiber-optic sleeve 180 inside the optical adapter 172. Tomaintain a high optical transmission during free rotation of theproximal end, while minimizing back reflection, the mating fiber-opticferrules 176, 180 are fabricated, preferably, with Ultra PhysicalContact (UPC) polished end faces. In some embodiments, separatedisposable optical adapter 172, which is supplied to the user as aseparate sterilized component, plugs into the probe interface of theFD-OCT system and remains connected throughout the procedure. At thecompletion of the procedure, the adapter and the probe are removed as aunit by the user for disposal.

To position the pressure probe 90′, the operator the proximal end of theprobe 90′ from the optical adapter 172, inserts the probe 90′ through aguide catheter into the artery, and steers the probe 90′ to the targetlocation using the removable torque handle 168 at the distal end. Oncethe wire has been positioned and any additional device has been insertedover the pressure probe, the operator re-inserts the proximal end of theprobe into the optical adapter 172 and locks the clamp 184 to keep thesurfaces of the optical fibers 176, 180 in close contact if the pressureprobe 90′ moves.

FIGS. 8( a-b) show yet another alternative construction of an opticalpressure probe 90″ that is suitable for intravascular pressuremeasurement with an FD-OCT system configured according to the variousembodiments of the invention. Unlike the pressure probes 90, 90′illustrated in FIGS. 6 and 7, this version of the pressure probe isdesigned for rapid delivery over a primary guide wire.

In many instances, especially when an artery is tortuous or otherwisedifficult to access, the clinician prefers to employ an independentprimary guide wire rather than to steer the unsupported pressure probeto the target site. The guide wire (not shown) is inserted at the probetip 190 and exits through the guide wire exit 194. The guide wire isinserted into the vessel and moved to the position of interest in thevessel. The pressure probe 90″ is next inserted into the vessel over theguide wire and also moved to the place of interest in the vessel. Theposition of the pressure transducer can be monitored under x-ray imagingusing the radio-opaque marker 198 located on the probe. The guide wiremay then be removed and the pressure measurements performed.

Eliminating the need for steerability of the pressure probe 90″ makesthe rapid-exchange pressure wire easier and less costly to fabricate;however, to minimize restriction of blood flow, its cross section shouldbe kept small. Therefore, to avoid inaccurate measurement of vascularresistance, the relatively large-diameter tip of the pressure probe mustbe placed far enough away from a tight vessel stenosis to preventfurther restriction of blood flow. To satisfy this constraint, in oneembodiment, the distance from the exit port of the guide wire to thepressure sensor (labeled ‘L’ in FIG. 8 a) is set to 2-4 cm. The offsetof the pressure sensor 10′ relative to the tip 190 enables the user toplace the largest-diameter segment of the probe outside of the stenosisduring the measurement of pressure both distal and proximal to thestenosis.

The utility and ease of use of the rapid-exchange version of thepressure probe can be improved by modifying its construction accordingto FIGS. 9( a-b). The distal end of the pressure probe 90″′ shown inthis figure includes a series of ports 160′ at evenly spaced intervals(typically 2-5 mm) that transmit the pressure at particular points alongthe axis of the artery to the inner lumen of the probe. An opticalpressure transducer 10′ at the tip of an optical fiber inside the lumenof the pressure probe senses the local pressure in the vicinity of thepressure ports. The optical fiber and attached pressure transducer aredesigned to translate longitudinally inside the lumen as the motorinside the probe interface pulls the fiber connector back at a constantspeed. Before use, the probe is flushed with saline. A liquid seal atthe proximal end between the optical fiber and the non-rotating shellover the body of the fiber-optic connector prevents the escape of fluidinto the probe interface.

To perform a pressure measurement, the clinician inserts the tip ofprobe 90′″ across the target lesion and pushes it forward until thetarget lesion lies between the radio-opaque markers 198′, 198″ on bothsides of the series of pressure ports. The measurement is initiated byactivating the automated pullback mechanism (part of the standard FD-OCTprobe interface), which pulls the transducer 10′ along the length of theprobe 90′″ lumen at a constant velocity adjacent the series ofpressure-sensing ports 160′. The pressure measured as a function of timeprovides a profile of the pressure across the lesion.

FIG. 9 d shows an embodiment of a multi-hole probe within the lumen of astenotic vessel. FIG. 9 e shows the measured pressure at variouspositions in the vessel corresponding to the cross-section in FIG. 9 dand the pressure values measured by the transducer at each of the holesin the probe. FIG. 9 f shows the pressure readings by the transducer asit is moved by the individual holes. Because the transducer determinesthe pressure downstream (distal) from it, a stepped pressure measurementis obtained as the transducer moves by the individual holes. Thepressure errors introduced by this technique are minimal.

FIG. 10 is a cross-sectional view of the tip of a catheter that combinesthe functions of an intravascular OCT imaging probe and a pressure wire.Depending on its construction, the combination catheter is compatiblewith the embodiments of the FD-OCT system shown in FIG. 2 and FIG. 5.The key feature of the combined catheter is the integration of thepressure transducer into the tip of the OCT imaging catheter. An opticalFabry-Perot pressure transducer 10″ is mounted on the distal end of ashort length (typically 1-2 mm) of coreless or large-core step-indexmultimode fiber 26′. The distal end 202 of the fiber segment is polishedat an angle of 40-50 degrees and coated with a thin dielectric ormetallic film. To avoid excessive back-reflection from the distal end ofa fiber-tip lens assembly 206, the fiber segment 26′ with the attachedtransducer is glued to the fiber-tip lens assembly 206 with an adhesive210 that matches the refractive index of the lens 206.

For use of the combination catheter with the FIG. 2 embodiment of theFD-OCT system in which both the OCT system and pressure transduceroperate in the same wavelength band (typically 1260-1360 nm), thethin-film coating on the angle-polished end of the fiber segment isselected to reflect a large fraction (75-90%) of the incident light atspecific wavelengths for OCT measurements and to transmit the remainingfraction to the transducer 10″ for pressure measurements. Sincereflection from the Fabry-Perot cavity of the pressure transducer 10″modulates the spectrum of the raw OCT interference signal a series ofartifactual lines appear in the OCT image at a depth proportional to themodulation frequency. To avoid degradation of the OCT image caused bythese lines, the zero point of the OCT interferometer can be set bysetting the delay in the interferometer such that the lowest frequencyof the displayed OCT signals exceeds the peak frequency of the spectralmodulation.

For use of the combination catheter with the FIG. 5 embodiment of theFD-OCT system in which the OCT system and the pressure transduceroperate in the first and second wavelength bands, respectively; thethin-film coating on the angle-polished end 202 of the fiber segment 26′is selected to reflect light maximally in the first wavelength band andto transmit light maximally in the second wavelength band. With the lens206 and pressure transducer 10″ attached to its tip, the optical fibermounts inside the lumen of torque wire 214 that rotates inside thecatheter sheath 218. The catheter sheath is filled from the proximalport with saline or contrast medium. During pressure monitoring, therotation of the torque wire 214 is turned off. The distal end of thecatheter can employ either a monorail tip for rapid-exchange delivery,as in the embodiments shown in FIG. 8 and FIG. 9, or a spring tip forinsertion in the artery without a guide wire, as shown in the pressurewire embodiments in FIG. 6 and FIG. 7.

Other beam-splitting arrangements at the catheter tip are also possible.For example, the fiber-tip lens assembly can be angle-polished andcoated, rather than the fiber attached to the transducer. Also, a bulkoptical component, such as miniature prism or mirror, can be employed asa beam splitter instead of an angle-polished optical fiber.

FIG. 11 shows a set of common-mode interference signals acquired from anoptical pressure probe connected to an FD-OCT system that was configuredaccording to the embodiment of the invention shown in FIG. 2, operatingin the pressure measurement mode. Acquired over a range of pressures(−40 mmHg<P<180 mmHg), the signals were recorded at successive opticalclock intervals, in proportion to the wavenumber or optical frequency ofthe wavelength-swept laser. In this example, the abscissa spans anoptical frequency range of 220-240 THz or, equivalently, a wavelengthrange of 1250-1360 nm. The low-frequency spectral modulation of thesignal originates from reflections within the main Fabry-Perot cavityfunned by the diaphragm and the body of the in the optical pressuretransducer. The width of the cavity of the transducer in this examplewas approximately 17 μm. As shown by the arrows in FIG. 11, thelow-frequency modulation pattern shifts to higher frequencies inproportion to pressure. The high-frequency spectral modulationsuperimposed on the signal was caused by reflection from a “parasiticcavity” in the transducer 10′ formed by the interface between the inputoptical fiber 26′ and the sensor body 22. Neither the frequency nor theamplitude of this parasitic modulation changes significantly withpressure.

The characteristics of the time-dependent interference signal generatedby the pressure transducer at the output of the photodetector (see forexample 120 in FIG. 4) of the FD-OCT system can be expressed as:V(t)=KP ₀(k)└r _(FP)(k,P)+r _(p)(k)┘  (1)

-   -   where K is a constant, P₀ (k) is the optical power incident on        the transducer; r_(FP) (k,P) and r_(p) (k) are, respectively,        the reflectivities of the Fabry-Perot and parasitic cavities of        the pressure transducer. The interference signal, power, and        transducer reflectivities are functions of the optical        wavenumber (k) of the light emitted by the laser, which varies        as an arbitrary function of time (t). In the FD-OCT system, the        signal voltage (V) from the photodetector 94 is sampled by the        analog-to-digital converter 50 at evenly spaced wavenumber        intervals, k_(n)=k₀+(n−1)Δk; here, k₀ is the initial wavenumber        of the laser sweep, Δk is the wavenumber sample interval, and        n=1, 2, . . . N, where N is the number of samples. According to        these definitions, the recorded digital pressure signal can be        expressed as an array of N values measured at successive optical        clock intervals (in proportion to wavenumber),        V _(n) =KP ₀(k _(n))[r _(FP)(k _(n) ,P)+r _(B)(k _(n))], for        n=0,1,2, . . . N  (2)

The reflectivity r_(FP) varies in relation to the pressure-dependentlength, L(P), of the Fabry-Perot cavity, according to:

$\begin{matrix}{{r_{FP}\left( {k_{n},P} \right)} = {{1 - \frac{1}{1 - {{r_{c}}{\sin\left\lbrack {2\; k_{n}{L(P)}} \right\rbrack}}}} = \frac{{r_{c}}{\sin\left\lbrack {2\; k_{n}{L(P)}} \right\rbrack}}{{{r_{c}}{\sin\left\lbrack {2\; k_{n}{L(P)}} \right\rbrack}} - 1}}} & (3)\end{matrix}$

Here, the magnitude of the effective reflection coefficient of thecavity, |r_(c)|, is approximately equal to the geometrical mean of themagnitudes of the reflection coefficients of the reflecting surfaces ofthe Fabry-Perot cavity. For most transducers, the length (L) decreasesapproximately linearly with pressure over a wide range of pressures. Theparasitic reflectivity, r_(B)(k), generated by M parasitic cavitieswithin the transducer's body or packaging, generates pattern noisecomposed of sinusoids of different frequencies,r _(p)(k _(n))=|r _(p1)| sin(2k _(n) l ₁)+|r _(p2)| sin(2k _(n) l ₂)+ .. . +|r _(pM)| sin(2k _(n) l _(M))  (4)

-   -   where |r_(p1)|, |r_(p2)|, . . . , |r_(pM)| are the magnitudes of        the effective reflection coefficients of the parasitic cavities        and l₁, l₂, . . . l_(m) are the lengths of the parasitic        cavities.

These three equations, 2, 3 and 4 represent a mathematical model of thesignal recorded by the FD-OCT system. The nominal Fabry-Perot cavitylength (L) at a given pressure in Equation 3 is known from themanufacturing process. The reflection coefficient |r_(c)| is determinedby fitting signals measured from a sampled number of pressuretransducers. In practice, a single parasitic cavity usually dominates,and its length and effective reflection coefficient can be determined byFourier transformation of pressure signals measured from the samplednumber of pressure transducers.

In accordance with the present invention, the algorithm for processingthe pressure signal proceeds according to following steps:

First the signal is normalized by dividing the recorded signal array ofvoltages (V_(n)) by the laser power to obtain the normalized signal:V _(n) ⁰ =V _(n) /P(k _(n))  (5)

Next the parasitic cavity noise is removed by applying a Butterworth orequivalent low-pass filter to the normalized signal (V_(n) ⁰) with acut-off frequency below that of the lowest frequency component of thereflection coefficients r_(p)(k). The result is:V_(n,F) ⁰=LPF{V_(n) ⁰}  (6)where LPF{ } represents the low-pass filtering operation. Next thespectral null, the sample wavenumber at which the amplitude of V_(n,F) ⁰is lowest, is detected. V_(n,F) ⁰ is first convolved with a templatearray of values proportional to r_(FP)(k), with |r_(c)| and L(P)determined by fitting filtered arrays measured from a sample oftransducers at reference pressures. The spectral null of V_(n,F) ⁰occurs at the array index n_(min) at which the amplitude of theconvolved is maximum. Alternatively, the minimum, maximum or steepestedge of V_(n,F) ⁰ can be located by conventional differentiation orgradient-search methods known to persons skilled in the art.

The spectral null is then tracked and unwrapped. If more than onespectral null of V_(n,F) ⁰ occurs within the pressure range of interestor nulls move out of the laser's wavelength band at the extremes of thepressure range, the position of nulls can be tracked across multiplelaser sweeps to extend the pressure measurement range. Tracking can beaccomplished by standard phase unwrapping techniques applied to asequence of stored V_(n,F) ⁰ array values.

FIG. 12 shows a calibration curve of a typical pressure transducer. Thecurve was obtained by applying the above algorithm to the raw pressuresignals in FIG. 11. Since the optical clock interval number at which thespectral null occurs, n_(min), varies approximately in linear proportionto the applied pressure, the pressure can be estimated accurately fromn_(min) once the slope and offset of the calibration curve are known. Inpractice, a polynomial function is used to fit the calibration curve ofan individual pressure probe or catheter and the stored coefficients areemployed to estimate pressure from measured n_(min) values.

FIG. 13 shows dynamic pressure waveforms measured by the same pressureprobe from which the raw signals in FIG. 11 were obtained. In thisexample, good correspondence compared to a commercial strain-gagetransducer was obtained by using a simple first-order polynomial(linear) calibration curve. FIG. 14 shows the pressure waveformsmeasured by the pressure probe across a tight stenosis in a simulatedartery. In this experiment, the pressure-sensing segment of the pressureprobe was pulled through the stenosis at a constant speed by a motor inthe probe interface. The sharp reductions in both mean and pulsatilepressures provide clear evidence of the flow resistance imposed by thestenosis.

The examples presented herein are intended to illustrate potential andspecific implementations of the invention. It can be appreciated thatthe examples are intended primarily for purposes of illustration of theinvention for those skilled in the art. There may be variations to thesediagrams or the operations described herein without departing from thespirit of the invention. For instance, in certain cases, method steps oroperations may be performed or executed in differing order, oroperations may be added, deleted or modified.

Variations, modification, and other implementations of what is describedherein will occur to those of ordinary skill in the art withoutdeparting from the spirit and scope of the invention as claimed.Accordingly, the invention is to be defined not by the precedingillustrative description, but instead by the spirit and scope of thefollowing claims.

What is claimed is:
 1. A probe for an optical coherence tomographysystem comprising: a body having a wall defining a bore; a first opticalfiber segment located within the bore, the first optical fiber segmenthaving a first end and a second end; a second optical fiber segmenthaving a longitudinal axis located within the bore, the second opticalfiber segment having an angled end and a optical pressuretransducer-facing end; an optical pressure transducer located within thebore and in optical communication with the first optical fiber segmentand the optical pressure transducer-facing end; and a coating disposedon the angled end, the coating configured to receive light and transmita first portion of the received light to the optical pressuretransducer, wherein the first optical fiber segment is positioned toreceive a first optical signal for optical coherence tomography imagingfrom the angled end, wherein the body further defines at least oneopening from the bore to the environment by which pressure from theenvironment is transmitted to the optical pressure transducer.
 2. Theprobe of claim 1 wherein the coating is configured to reflect a secondportion of the received light through the wall at an angle relative tothe longitudinal axis.
 3. The probe of claim 2 wherein the angle rangesfrom 40 degrees to 50 degrees.
 4. The probe of claim 1 furthercomprising an adhesive disposed between the angled end and a fiber-tiplens assembly.
 5. The probe of claim 1 wherein the body is a polymertube.
 6. The probe of claim 1 wherein the coating reflects light in afirst wavelength band and transmits light in a second wavelength band.7. The probe of claim 1 wherein the coating reflects between 75% to 90%of incident light through the wall and transmits between 10% to 25% ofthe incident light to the optical pressure transducer.
 8. The probe ofclaim 1 further comprising a torque cable disposed in the bore of thebody.
 9. The probe of claim 1 wherein an angle of the angled end rangesfrom 40 degrees to 50 degrees.
 10. The probe of claim 1 wherein thefirst optical fiber segment is positioned to receive a second opticalsignal for pressure measurements from the optical pressure transducer.11. A combination probe for an optical coherence tomography systemcomprising: a body having a wall defining a bore; an optical fiberhaving a longitudinal axis located within the bore, the optical fiberhaving a first end and a second end; a partial reflector located withinthe bore and positioned to reflect a first portion of light receivedfrom the first end of the optical fiber through the wall at an anglerelative to the longitudinal axis, wherein the first end of the opticalfiber is positioned to receive scattered light passing through the bodyand transmit the scattered light to the optical coherence tomographysystem for interferometric imaging; and an optical pressure transducerlocated within the bore and positioned to receive a second portion oflight transmitted through the partial reflector, wherein the bodyfurther defines at least one opening from the bore to the environment bywhich pressure from the environment is transmitted to the opticalpressure transducer.
 12. The probe of claim 11 wherein the partialreflector comprises a coating.
 13. The probe of claim 12 wherein thecoating reflects light in a first wavelength band and transmits light ina second wavelength band.
 14. The probe of claim 12 wherein the coatingreflects between 75% to 90% of incident light through the wall andtransmits between 10% to 25% of the incident light to the opticalpressure transducer.
 15. The probe of claim 11 wherein the body is apolymer tube.
 16. The probe of claim 11 wherein the angle ranges from 40degrees to 50 degrees.